This application claims the priority of japanese Patent Applications Nos. 6-168977 and 6-168978 filed on Jun. 28th, 1994.
1. Field of the Invention
The present invention relates to an electronic endoscopic device, and more particularly, to electronic shutter control of a solid-state image pickup device in an electronic endoscopic device.
2. Description of the Related Art
An electronic endoscopic device has an electronic shutter function for controlling the storage time of a charge as pixel information for a CCD (Charge Coupled Device) which is a solid-state image pickup device. This electronic shutter function can control exposure in image forming instead of ALC (Automatic Light Control) using a stop member and the like, which has been adopted hitherto. In other words, while the ALC detects the luminance of an image signal and controls the amount of incident light by a stop member or the like, the electronic shutter function can obtain an image having the optimum brightness by controlling the amount of charge to be stored in the solid-state image pickup device based on luminance information of an image signal, that is, by decreasing the charge storage time when an image is bright, and increasing it when the image is dark.
However, in the above-mentioned electronic shutter control, since control signals for charge storage, that is, a discharge pulse and a read pulse are generated during a blanking period of a horizontal synchronizing signal, exposure control accuracy is lowered and exposure errors occur when a subject is bright in, for example, a close-up.
FIG. 8 is a waveform chart showing a conventional control operation. In a control device, for example, 262.5 horizontal synchronizing signals shown in FIG. 8(B) are generated during a vertical scanning period (1V) shown in FIG. 8(A). If a control signal or the like is formed during an effective period of the horizontal synchronizing signal, noise appears in an image, and therefore, a control pulse for an electronic shutter is formed during each blanking period B of the horizontal synchronizing signals. In an example shown in FIG. 8(C), discharge pulses Pa are formed in blanking periods B of horizontal synchronizing signals 1 to 131 based on an exposure control signal, and a read pulse Pb is formed in the last blanking period B and a blanking period of a vertical synchronizing signal. As shown in FIG. 8(D), a charge stored in elements of the CCD is once discharged by the above discharge pulses Pa, and then, the charge storage is started again. A stored charge denoted by numeral 100 is read out by the read pulse Pb.
In the above case of FIGS. 8(C) and (D), the leading end of the electronic endoscope is not close to the subject and exposure time is set long. In this case, even if the last discharge pulse Pa is output during a blanking period B of the previous horizontal synchronizing signal 130 shown by a broken line, an added charge 101 has only a little influence on the charge 100. However, as shown in FIGS. 8(E), (F) and (G), if the leading end of the endoscope is close to the subject and the exposure time is short, that is, when discharge pulses Pa have been formed up to a blanking period B of a horizontal synchronizing signal 260, the amount of a stored charge 102 is small. Therefore, an added charge 103 stored after a discharge pulse Pa of the previous horizontal synchronizing signal 259 shown by a broken line has a great effect on the stored charge 102. Accordingly, in a close-up or the like, exposure accuracy in the electronic shutter control is lowered, and more exposure errors arise than in cases other than the close-up.